The invention relates to wax-coated, silica-matting agents and their application in the matting of radiation curable systems.
As noted in WO97/08250, radiation curable, e.g., ultraviolet (UV) curable, compositions are used to coat a variety of surfaces. These compositions provide a number of advantages, of which include rapid curing, superior durability, chemical resistance and storage stability. However, they also result in relatively hard, smooth and glossy coatings. As these coatings become more widely used, there are more and more instances where it is preferable to reduce the gloss of these coatings. Reducing this gloss is also known as matting. Matted surfaces provide the finished articles with a more pleasing appearance, as well as hide imperfections at the surface, especially in wood, furniture, and several other applications.
Silica is one example of an additive used to matt glossy surfaces. Silica matting agents are used in both solvent and water based finishes to reduce or control gloss. However, lacquer systems with a low VOC content are becoming more and more important, due to national and international environmental and legislative pressures. Indeed, in some instances the VOC content in coatings is reduced to substantially zero, i.e., these coatings have a substantially 100% solids content. As a result, films prepared from these coatings may shrink by only 5 to 15% upon curing. This low shrinkage prevents conventional silica matting agents from efficiently matting at their conventional levels. For example, silica matting agents which work effectively in lacquer systems in which the film shrinks up to 70% as the film""s solvent evaporates, are not able to matt high solids, radiation (EB, UV) curing systems efficiently. Specifically, the film thickness of a UV curable coating is not reduced to a point the silica particles can cause deformations on the film""s surface.
In addition, a matting agent""s effectiveness in radiation curable coatings is affected by the time necessary for curing the coating. Fast curing compositions are more difficult to matt using conventional agents. Today a wide range of radiation-curable lacquers are available. Most lacquers are based on combinations of acrylate functional prepolymers (oligimers) and monomers, together with photoinitiators in the case of UV curing. The curing time of these systems is dependent on their composition and on the processing environment. Therefore, both the lack of film shrinkage and short curing time in certain radiation curable compositions (fractions of a second) hinders the efficacy of matting agents.
Special techniques such as xe2x80x9cdual curexe2x80x9d methods have been used to matt UV curable systems. These techniques involve a two stage (hence xe2x80x9cdualxe2x80x9d) cure process, designed to encourage the formation of a microrough surface, either through improved orientation of the matting agent particles towards the film surface or through surface wrinkling. An example of the former is xe2x80x9cGradient Intensity Curexe2x80x9d process involving a pregelling cure stage followed by a final surface cure stage. These techniques have the disadvantages of requiring specially designed equipment and formulations, as well as being notoriously difficult to control.
Another technique to reduce gloss in these coatings is to use higher levels of matting agents. However, high dosages of silica matting agent increases cost and also has a negative impact on lacquer properties, e.g., rheology or optical properties of the cured film.
The pore volume of the matting agents can be modified to reduce affects on rheology. However, lowering the pore volume of conventional micronized amorphous silica agents results in particles of higher apparent density, which in turn results in fewer particles per unit volume in the lacquer formulation. Because the number of particles have a direct influence on matting efficiency, an amorphous silica matting agent with a low pore volume exhibits a lower matting effect than a matting agent with a higher pore volume of the same particle size distribution.
Coarser or larger particles could also increase the gloss reduction of the cured film. However, increasing the particle size of the micronised product can adversely increase the surface roughness of the lacquer film to an unacceptable degree.
It is also known that with increasing film thickness and using coating compositions having higher cure rates, matting becomes increasingly more difficult. In those situations, matting can be maintained to a certain extent by increasing the amounts of matting agent. Increasing the number of micronized particles, however, dramatically and adversely changes the rheological properties of the lacquer.
In summary, due to the fast curing of UV-systems and the difficulties of matting such systems, it has been a practice to choose a micronized matting agent with a mean particle size which is close to the thickness of the film after it cures. Accordingly, when one is using a particular coating, the matting agent that can be used is one that has a particle size close to the film thickness of the resulting coating.
It is therefore desirable to have a matting agent which is efficient in radiation curable coatings and can be used at relatively low levels so that viscosity of the formulation is not adversely affected. It is also desirable to have a matting agent that is efficient for fast and slow curing systems, thereby offering the user of the matting agent greater flexibility. In other words, it would be desirable to have available a matting agent that produces stable, reproducible low gloss coatings for a variety of coating formulations without the usual problems of unworkable viscosity increases, inconsistent gloss values at varying coating weight and avoids the use of special application techniques to reduce gloss. In addition, it is also desirable that the agent produces a consistent matting effect over a wide range of thicknesses.
The aforementioned objects are unexpectedly obtained by employing silica matting agents having a maximum pore volume of 1.4 ml/g and a wax content of at least 15% and maximum of 30% by weight. The wax content typically is in the range of 18 to 22% by weight. The wax preferably has a melting point in the range of 60-120xc2x0 C., and most preferably in the range of 60-90xc2x0 C. The matting efficiency of the agent is also affected by the particle size of the invention. The invention has a median particle size in the range of 2.0-12.0 xcexcm, with a preferred range of 2.0-5.0 xcexcm. It is also unexpected that the matting agents having particles sizes in the lower part of above ranges can enhance matting efficiency further without significant adverse affects on the viscosity of the coating composition.
The wax coated silica-matting agents can be used with a variety of radiation curable compositions and can be manufactured by simultaneously melting and milling the wax and silica to the desired particle size. The milling process is preferably carried out in a fluid energy mill with an inlet temperature which is above the melting point of the used wax.